The invention concerns a method and an apparatus for detecting a movement of a surface which is irradiated with a coherent light beam emitted especially from a laser wherein variations caused by said movement in a speckle pattern of the reflected light beam are detected.
Such a method and system are known from WO 79/00841 and are used for the measurement of the vibration of an eardrum, wherein the vibration of the eardrum are caused by the application of successively different sound frequencies. The known detector which is placed near the irradiated surface is able to measure exposed structures.
In ophthalmology, it is known (WO 01/91661 A1) to use laser systems in the therapy of retinal diseases, especially of dysfunctional retinal pigmentary epithelium (RPE). During the irradiation, the targeted disease areas of the eye fundus can be thermally sclerosed and, as a result of the subsequent regeneration and lateral proliferation of RPE cells in the sclerotic zones, a substantial restoration of the intact RPE is possible. The selective RPE therapy (SRT) avoids damage to the RPE environment.
In the application of SRT, a burst, i.e. a pulse sequence, of approximately 30 laser pulses with a pulse duration, in each case, of 1.7 μs in the green spectral range and with a pulse sequence rate of 100 Hz at a wavelength of 527 nm is used. Natural and numerous variations to these treatment parameters are also possible. For the thermotherapy of biological tissue, particularly of the eye fundus, clear preference is given to pulse durations of few microseconds. As a result of the strong pigmentation of the RPE approximately 50% of the incident light in the green spectral range is absorbed by the pigment granules (melanosomes) in the RPE cells. High temperatures occur in the RPE in the case of corresponding irradiation (approx. 600 mJ/cm2 per pulse), which lead to intracellular microvaporization on the strongly heated RPE melanosomes.
Resulting micro-bubbles increase the cell volume for microseconds and in all probability ultimately lead to the disruption and disintegration of the RPE cells. The irradiation threshold for cell damage can drop significantly through the application of multiple pulses. There is a considerable variation between patients concerning the prerequisites for laser therapy (e.g. transparency of the lens or glass body, pigmentation of the retina) in connection with eye fundus treatment. Research results show that the necessary pulse energies for producing RPE effects vary intra-individually by up to 100% and inter-individually to an even greater extent. Existing experience shows that the pulse energy must be no more than a factor of two above the threshold pulse energy for producing RPE damage, or otherwise visible damage recurs to the retina.
EP 1 643 924 B1 describes a dosimetry control which detects the thermally induced bubble formation in the irradiated tissue to provide a signal for adjusting the energy of the light beam irradiating the tissue. The detecting device can be either a pressure transducer or a piezo-ceramic pressure sensor or a photo-detector.
The problem to be solved by the invention is to provide a method and a system to detect variations at the surface of an object irradiated by a coherent light beam with a small technical expenditure.
The invention provides a method for detecting a movement of a surface on an irradiated sample, especially on a biological tissue at the fundus of an eye. The method comprises the steps of irradiating said surface with a coherent light beam, detecting variations caused by said movement in a speckle pattern produced by reflections of said light beam at said surface, selecting a single speckle from said speckle pattern, and detecting said variations at the selected speckle.
The invention provides a system for detecting a movement of a surface on an irradiated sample, especially on a biological tissue at the fundus of an eye. The system comprises a light source, especially a laser, emitting a coherent light beam, a light guiding path along which the coherent light beam is directed onto said surface, and a detector, especially a photo-detector adapted to detect variations caused by said movement of the surface at a single speckle selected from a speckle pattern produced by the light beam reflected from the irradiated surface.
The coherence length of the irradiating light beam is greater than the optical path length difference to be measured. For irradiating the surface especially a surface at the fundus of an eye, the irradiating light beam and the light beam reflected at the irradiated surface are guided through a confocal light path. Preferably, the irradiating light beam and the reflected light beam are guided by means of a multi-mode optical fiber, which is intrinsic confocal with the irradiated area so that all points of the irradiated spot are imaged back into the optical fiber without any additional adjustment. The reflected light beam can be split from the irradiating light beam for the detection of especially the variations of the intensity of the reflected light at the selected speckle. The variation of the speckle corresponds to changes at the irradiated area. The measurement of the speckle variations is performed phase-sensitively preferably by means of a photo-detector, especially fast photodiode. It is possible to measure very small changes on the irradiated area, for instance in the magnitude of the wavelength of the irradiating light.
Preferably, the frequency of the detected variation at the selected speckle is correlated to the velocity of a change, especially of the movement at the irradiated surface. This correlation enables to conclude to the origin which caused the variation or the change at the irradiated area, especially if the change or the movement of the surface is caused by the irradiating light beam.
The invention is preferably used to perform a therapy of eye diseases by means of selective retina therapy (SRT). SRT performs a selective microphotocoagulation for degradation of the retinal pigmentary epithelium (RPE), wherein a laser, especially a frequency-doubled Nd:YAG-laser, emitting short pulses, for instance of 1.7 μs or from 1 μs to few μs, in the green space, for instance at a wavelength of 527 nm is used. The fundus of the eye is irradiated with a spot size of about 200 μm. The incident light is absorbed by the melanosomes of the RPE, which leads to an intracellular microvaporization on the heated RPE melanosomes and the creation of micro-bubbles which cause damage to the cells within the irradiated area. This selective cell damage initiates proliferation and migration of RPE-cells from the environment in the damaged area to obtain an irradiation-induced regeneration of the RPE.
The invention enables an accurate dosimetry for the blind treatment of the selective RPE therapy (SRT) by the immediate on-line detection of the bubble formation which can be achieved just after a single pulse within SRT-pulse sequence of for instance about 30 pulses.
During the irradiation, the local heating produces a thermoelastic expansion of the heated tissue. If the formation of micov-bubbles occurs during the irradiation, the velocity of the changes or variations of movement of the irradiated surface are quicker than the thermoelastic expansion of the heated tissue. The changes of the optical path length created by the cavitation bubble and the movement of the surface of the cavitation bubble enables a phase sensitive detection of the bubbles which appear as fast varying signal components. This ability also offers the possibility to detect thermal expansions of cell components, before cavitation bubbles occur. The speckles of the reflected light beam, especially the detected light intensity varies accordingly with an increased frequency within the MHz-region. The frequency of the variation of the light intensity in the speckle pattern, especially of the selected single speckle is correlated with the velocity of the changes at the irradiated area and can provide indication of the bubble-formation.
In order to achieve an adjustment of the energy for the irradiating light beam, one or more test irradiations causing the movement of the illuminated surface can be performed and the energy of the irradiating light beam is determined, when a predetermined variation at the selected speckle occurs.
To achieve an accurate dosimetry for the selective retina therapy (SRT), one or more test irradiations, starting with an energy for the irradiating light beam below the therapeutical window used for SRT can be performed with increasing energy. When the bubble formation is indicated by the predetermined variation, especially the frequency of light intensity variations at the selected speckle, the lower limit of therapeutical energy window within which SRT can be conducted is achieved. The SRT can then be started automatically or by the physician.
In order to compensate the influence of sources disturbing the measurement, the total intensity of the reflected light at all speckles of the speckle pattern can be integrally measured additionally and the modified intensity of light measured at the selected speckle can be normalized with respect to the total intensity.
Additionally to the above explained SRT, the invention can be applied at the photocoagulation of the retina. U.S. Pat. No. 6,830,567 B2 discloses a non-invasive temperature determination during the photocoagulation of the retina, wherein the tissue expansions are measured by means of an optical or a pressure (acoustical) sensor. The present invention provides an alternative measurement of temperature during the retina photocoagulation. The expansion of the tissue is in the region of 100 μm which is within the measurement sensitivity of the inventive detecting system. The thermal expansion of the treated retinal area can be determined by the variation of the light intensity at the selected single speckle and therefrom the average temperature within the irradiation spot can be calculated. The above explained online-dosimetry can be used for the retina coagulation as well.
Generally, the invention can be used for the optical measurement of sample variations having a high time resolution, for instance with respect to path length, distance, expansion, variations of scattering in the sample volume, wherein the measured sample has an at least slightly scattering or reflecting surface or sample volume to form a speckle pattern.